A method for increasing storage density and capacity in automated tape libraries is to stack tape cartridges in depth, one behind the other. The slots that hold these tape cartridges can be described as deep storage slots that allow for more than one tape cartridge to be stored in depth. Deep storage slots require a pushing mechanism to push a tape cartridge to the front opening of the deep storage slot as a front tape cartridge in order to position the front tape cartridge where a library robotic accessor can pick the front tape cartridge for placement into a tape drive or another library slot. An issue to be addressed for retaining tape cartridges in a deep storage slot is to ensure the push of the front tape cartridge to the front of the deep storage slot does not push the front tape cartridge out of the deep storage slot and onto the library floor. This is particularly relevant to pushing mechanisms that have a continuous force pushing the tape cartridge(s) toward the deep storage slot opening, such as, for example, by a spring or by gravity.
A widely used known solution for cartridge retention in tape libraries is to have a snap that engages in a notch on a side of a single tape cartridge within a tape slot. This snap is usually an integral part of the plastic slot material and snaps into a retention recess in a particular tape cartridge. This method of retention is an adequate solution for retaining a single tape cartridge, but is deficient when deep storage slots with multiple cartridges are used. The reason is that the retention snap must provide enough retention force to prevent the tape cartridge(s) from being pushed out of the slot by the continuous force mechanism. For the deep storage slot to function properly, when the front tape cartridge in a deep storage slot is removed the remaining tape cartridge(s) in the deep storage slot must be pushed forward to the slot opening. If the retention snap can prevent a single tape cartridge from being pushed out of the cell, then this snap also provides enough force to prevent the other tape cartridge(s) from moving past the snap at all toward the slot opening. In other words, the retention snap defeats the pushing mechanism for allowing the tape cartridge(s) to be positioned at the front of the deep storage slot for cartridge access.
Another known solution uses internal retaining hooks to retain tape cartridges in a deep storage slot with a continuous force pushing the tape cartridges out of the cell. A problem that has been seen with this solution is that when the retaining hooks have a flexible force that is movable by a cartridge accessor, this force becomes very light and the continuous force that moves the tape cartridges can overcome these retaining hooks, and push the tape cartridges through the hooks out of the slot. In order for this to work effectively the strength of the retaining hooks must be significantly high. In order for the cartridge accessor to push these hooks open, it must be very robust and have significant power that is cost prohibitive. A further complication with this solution is that when a tape cartridge is being removed from the deep storage slot by the cartridge accessor, the continuous force pushes the tape cartridges to the front on the deep storage slot simultaneously. Because these tape cartridges are being pushed together, there is little to no space between the tape cartridges in order for the retention hooks to engage the second cartridge reliably.